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          Mysql事务隔离
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        <h1 id="Mysql事务隔离"><a href="#Mysql事务隔离" class="headerlink" title="Mysql事务隔离"></a>Mysql事务隔离</h1><p>SQL 标准的事务隔离级别包括：读未提交（readuncommitted）、读提交（read committed）、可重复读（repeatable read）和串行化（serializable ）</p>
<h3 id="读未提交"><a href="#读未提交" class="headerlink" title="读未提交"></a>读未提交</h3><p>一个事务还没提交时，它做的变更就能被别的事务看到，非常松的隔离方式</p>
<h3 id="读提交"><a href="#读提交" class="headerlink" title="读提交"></a>读提交</h3><p>一个事务提交之后，它做的变更才会被其他事务看到。</p>
<h3 id="可重复读"><a href="#可重复读" class="headerlink" title="可重复读"></a>可重复读</h3><p>一个事务执行过程中看到的数据，总是跟这个事务在启动时看到的数据是一致的。当然在可重复读隔离级别下，未提交变更对其他事务也是不可见的</p>
<h3 id="串行化"><a href="#串行化" class="headerlink" title="串行化"></a>串行化</h3><p>顾名思义是对于同一行记录，“写”会加“写锁”，“读”会加“读锁”。当出现读写锁冲突的时候，后访问的事务必须等前一个事务执行完成，才能继续执行</p>
<p>以下是一个例子，假设数据表T只有一列，其中一行的值为1，按照时间顺序执行两个事务A，B</p>
<figure class="highlight plaintext"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line">mysql&gt; create table T(c int) engine=InnoDB;</span><br><span class="line">insert into T(c) values(1);</span><br></pre></td></tr></table></figure>

<table>
<thead>
<tr>
<th>事务A</th>
<th>事务B</th>
</tr>
</thead>
<tbody><tr>
<td>启动事务，查询得到值1</td>
<td>启动事务</td>
</tr>
<tr>
<td></td>
<td>查询得到值1</td>
</tr>
<tr>
<td></td>
<td>将1改成2</td>
</tr>
<tr>
<td>查询得到值V1</td>
<td></td>
</tr>
<tr>
<td></td>
<td>提交事务B</td>
</tr>
<tr>
<td>查询得到值V2</td>
<td></td>
</tr>
<tr>
<td>提交事务A</td>
<td></td>
</tr>
<tr>
<td>查询得到值V3</td>
<td></td>
</tr>
</tbody></table>
<p>若隔离级别是“<strong>读未提交</strong>”， 则 V1 的值就是 2。这时候事务 B 虽然还没有提交，但是结果已经被 A 看到了。因此，V2、V3 也都是 2</p>
<p>若隔离级别是“<strong>读提交</strong>”，则 V1 是 1，V2 的值是 2。事务 B 的更新在提交后才能被 A看到。所以， V3 的值也是 2。</p>
<p>若隔离级别是“<strong>可重复读</strong>”，则 V1、V2 是 1，V3 是 2。之所以 V2 还是 1，遵循的就是这个要求：事务在执行期间看到的数据前后必须是一致的。</p>
<p>若隔离级别是“<strong>串行化</strong>”，则在事务 B 执行“将 1 改成 2”的时候，会被锁住。直到事务 A 提交后，事务 B 才可以继续执行。所以从 A 的角度看， V1、V2 值是 1，V3 的值是 2。</p>
<h3 id="事务隔离的实现"><a href="#事务隔离的实现" class="headerlink" title="事务隔离的实现"></a>事务隔离的实现</h3><p>在 MySQL 中，实际上每条记录在更新的时候都会同时记录一条回滚操作。记录上的最新值，通过回滚操作，都可以得到前一个状态的值。假设一个值从 1 被按顺序改成了 2、3、4，在回滚日志里面就会有类似下面的记录</p>
<p><img src="/../../../%E7%A0%94%E4%B8%80%E4%B8%8B/mysql45%E8%AE%B2/%E7%AC%94%E8%AE%B0/mysql4.JPG"></p>
<p>当前值是 4，但是在查询这条记录的时候，不同时刻启动的事务会有不同的 read-view。如图中看到的，在视图 A、B、C 里面，这一个记录的值分别是 1、2、4，同一条记录在系统中可以存在多个版本，就是数据库的多版本并发控制（MVCC）。对于 read-view A，要得到 1，就必须将当前值依次执行图中所有的回滚操作得到。同时你会发现，即使现在有另外一个事务正在将 4 改成 5，这个事务跟 read-view A、B、C 对应的事务是不会冲突的</p>
<p>基于上面的描述，我们要尽量避免使用长事务，长事务意味着系统里面会存在很老的事务视图。由于这些事务随时可能访问数据库里面的任何数据，所以这个事务提交之前，数据库里面它可能用到的回滚记录都必须保留，这就会导致大量占用存储空间</p>
<h3 id="事务的启动方式"><a href="#事务的启动方式" class="headerlink" title="事务的启动方式"></a>事务的启动方式</h3><p>mysql的事务启动方式有如下两种:</p>
<ol>
<li><p>显式启动事务语句， begin 或 start transaction。配套的提交语句是 commit，回滚语句是 rollback。</p>
</li>
<li><p>set autocommit&#x3D;0，这个命令会将这个线程的自动提交关掉。意味着如果你只执行一个 select 语句，这个事务就启动了，而且并不会自动提交。这个事务持续存在直到你主动执行 commit 或 rollback 语句，或者断开连接</p>
</li>
</ol>
<p>有些客户端连接框架会默认连接成功后先执行一个 set autocommit&#x3D;0 的命令。这就导致接下来的查询都在事务中，如果是长连接，就导致了意外的长事务。因此，我会建议你总是使用 set autocommit&#x3D;1, 通过显式语句的方式来启动事务</p>

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